1. Technical Field
The present invention relates to an antenna device and a radio communication apparatus for use in radio communication, particularly to an antenna device and a radio communication apparatus to be used for a wireless set designed to simultaneously perform transmission and reception of electromagnetic waves.
To be more specific, the present invention relates to an antenna device and a radio communication apparatus utilized in a back scatter type radio communication system for performing data communication by utilizing modulation of a reflected wave, based on transmission of an unmodulated carrier wave from the side of a reflected wave reader, an operation of changing over the antenna load impedance on the side of a reflector, etc., and particularly to an antenna device and a radio communication apparatus configured in a thin form by disposing a radiating conductor and a ground conductor plate oppositely to each other with an insulating substance interposed therebetween.
2. Background Art
By putting a plurality of apparatuses in network connection, it is possible to realize enhancement of efficiency of command and data transmission, sharing of information resources, and sharing of hardware resources. In recent years, furthermore, radio communication has been paid attention to as a system for librating the users from wiring based on a wired system.
Examples of standards as to radio communication include IEEE (The Institute of Electrical and Electronics Engineers) 802.11, HiperLAN/2, IEEE 802.15.3, Bluetooth communication, and so on. In recent years, wireless LAN has been markedly spread, since wireless LAN systems have come to be inexpensive and to be incorporated in PCs in a standardized manner.
Radio communication systems on a comparatively small scale are used for data transmission between a host apparatus or apparatuses and a terminal apparatus or apparatuses in homes or the like. Here, examples of the host apparatus include stationary type come electronic products such as television, monitor, printer, PC, VTR, DVD player, etc. On the other hand, examples of the terminal apparatus include mobile apparatuses the power consumption of which is suppressed as much as possible, such as digital camera, video camera, cellular phone, PDA, portable type music reproduction device, etc. An example of application of this kind of system is uploading of image data picked up by a cellular phone with camera or a digital camera into a PC through wireless LAN.
However, since wireless LAN in itself has been designed and developed on the assumption that it is used in computers and, therefore, its power consumption becomes a problem where it is mounted in a mobile apparatus. Most of the wireless LAN cards of the IEEE802.11b type commercially available at present have a power consumption of not less than 800 mW at the time of transmission and not less than 600 mW at the time of reception. This level of power consumption means a heavy load to a battery-driven portable apparatus.
Even where a wireless LAN function is operated within short distances only so as to reduce the transmission power needed, the power consumption can be reduced by no more than about 80%. Particularly, transmission from an image input unit such as a digital camera to the image display unit side takes such a communication form that the transmission ratio occupies most of the whole communication, so that a radio transmission means further reduced in power consumption is demanded.
Besides, as for the Bluetooth communication, the transmission speed is as low as 720 kbps at maximum, inconveniently leading to a considerable time needed for transmission of images increased in file size attendant on the recent enhancement of image quality.
On the other hand, according to the radio transmission utilizing a reflected wave based on the back scatter system used in RFID, a lower power consumption can be realized even in such a communication form that the transmission ratio occupies most of the communications between apparatuses, for example.
A radio communication system of the back scatter type is composed of a reflector for transmitting data by a reflected wave having been modulated, and a reflected wave reader for reading the data from the reflected wave coming from the reflector. At the time of data transmission, the reflected wave reader transmits an unmodulated carrier wave. On the other hand, the reflector performs a load impedance operation such as turning ON/OFF of the terminal of the antenna, for example, and applies to the unmodulated carrier with a modulating treatment according to the data to be transmitted, to thereby transmit the data. Then, on the reflected wave reader side, the reflected wave is received and subjected to a demodulating and decoding treatment, whereby the transmitted data can be obtained.
In a reflected wave transmission system, an antenna switch for back scattering is composed generally of gallium arsenic IC, of which the power consumption is not more than several tens of microwatts. As for the average power at the time of data transmission, data can be transmitted with a power of not more than 10 mW in the case of delivery certification system, and with a power of several tens of microwatts in the case of one-way transmission. This means an overwhelming performance difference, as compared with the average power consumption of a general wireless LAN (refer to, for example, Japanese Patent Application No. 2003-291809).
FIG. 7 schematically shows the manner of radio data transmission based on the back scatter system used in RFID or the like.
In the back scatter system shown in the figure, an unmodulated carrier wave 707 is first transmitted from an antenna 704 of a host apparatus 701, and is received by an antenna 706 of a terminal apparatus 705. In this case, the terminal apparatus 705 applies a terminating operation to the antenna 706 according to a bit string of the data to be transmitted from the terminal apparatus 705 to the host apparatus 701, thereby producing a modulated reflected wave 708, which is transmitted toward the host apparatus 701. In the host apparatus 701, the modulated reflected wave 708 is received by the antenna 704, and data demodulation is conducted by a receiving unit (Rx) 703.
Thus, in the back scatter system, the host apparatus 701 simultaneously performs transmission of an unmodulated carrier wave 707 and reception of the modulated reflected wave 708 reflected by the terminal apparatus 705.
The unmodulated reflected wave transmitted from the host apparatus is attenuated in the going (forward) path until reaching the terminal apparatus 705, and is further attenuated upon at the time of reflection on the terminal apparatus 705 side and in the returning (backward) path until the reflected wave reaches the host apparatus 701. Therefore, the receiving unit 703 must treat the reflected wave which is low in power magnitude. In other words, the process in the receiving unit 703 is susceptible to influences of DC offset and transmitter noise, which makes it difficult to extend the transmission distance.
Here, one of the elements influencing the reception sensitivity of the host apparatus 701 lies in the phenomenon in which a part 710 of the unmodulated carrier wave transmitted from the transmitting unit 702 goes round to the receiving unit 703 in the course of the signal path inside the host apparatus 701. Since the frequency of the unmodulated carrier wave transmitted from the transmitting unit 702 and the frequency of the reflected wave received by the receiving unit 703 are in the same frequency band, the process in the receiving unit 703 is influenced by the transmitted signal (in this case, the unmodulated carrier wave) coming round from the transmitting unit 702 side.
The transmitted signal 710 coming round to the receiving unit 703 serves as a jamming noise to the modulated reflected wave 709 received at the antenna 704, and may induce a marked degradation of bit error rate (BER). Therefore, in the host apparatus 701, it is necessary to suppress the going-round of the transmitted signal 710 to the receiving unit.
FIG. 8 shows a configuration example wherein the going-round of a transmitted signal 811 to a receiving unit (Rx) 803 is improved by providing a circulator 810 at an antenna terminal of a host apparatus 801. However, enlarging the isolation of the circulator 810 generally raises the cost and enlarges the installation space. Besides, the going-round of the transmitted signal can be reduced to a certain extent by the circulator 810, but the value of the reduction is not infinite, and a practical value of isolation is about 20 dB.
In addition, FIG. 9 shows a configuration example in which the going-round of a transmitted signal 910 to a receiving unit 903 is improved by providing independent antennas 904 and 905 respectively at a transmitting unit (Tx) 902 and a receiving unit (Rx) 903 of a host apparatus 901. In this case, by a contrivance as to the method of laying out the antennas 904 and 905, it is possible to secure isolation between transmission and reception. However, since the antennas must be laid out in the state of being physically spaced from each other, a casing in which to mount the host apparatus 901 would necessarily be enlarged in size.
On the other hand, in a back scatter communication system designed to carry out reflected-wave transmission, antenna directivity is demanded at a reflected wave reader and a reflector. This point will be described in comparison to other radio communication systems.
In a general radio communication system such as wireless LAN, an electromagnetic wave transmitted from a control station such as an AP (access point) is received by an antenna of a terminal station. In the case of a system for carrying out somewhat long distance communication, as shown in FIG. 15, not only a direct wave coming from an AP but also scattered waves reflected by a wall and the like (multipass #1, multipass #2) are received on the terminal station side (over-the-horizon (OTH) communication). Since the multipass waves arrive at the terminal station after being reflected by a wall and the like, their polarization would be different from the polarization at the time of transmission from the AP (even when a vertically polarized wave is transmitted, the multipass waves may not necessarily be vertically polarized waves). Accordingly, a circular polarization or non-directional antenna is frequently used as an antenna on the terminal side.
On the other hand, in a reflected wave transmission, communication within comparatively short distances is presumed, so that an antenna at a reflector receives only a direct wave (in this case, an unmodulated carrier wave) coming from an antenna at a reflected wave reader, as shown in FIG. 16 (non-OTH communication). Here, it is assumed that a wave is transmitted with vertical polarization from the antenna of the reflected wave. In this instance, the transmitted wave cannot be favorably received unless the antenna 2 on the reflector side is an antenna capable of dealing with vertical polarization. Therefore, antennas with the same polarization are used for both the reflected wave reader and the reflector. As a result, the reflected wave produced in the reflector is transmitted as a vertically polarized wave to the reflected wave reader.
Besides, in the back scatter system, a carrier generation source is not provided on the reflector side, and the electromagnetic wave received is reflected in carrying out data transmission; due to this principle, the signal magnitude is very low and, further, it is attenuated in both the going (forward) path and the return (backward) path of the electromagnetic wave. Therefore, for permitting the unmodulated carrier wave to reach the reflector efficiently and for receiving the reflected wave efficiently, it is desired that the antenna of the reflected wave reader and the reflector have directivity toward each other so as thereby to obtain a high antenna gain.
Here, as an antenna having directivity, there is known a planar patch antenna (also called MAS (Micro Strip Antenna)). The patch antenna is a thin antenna configured by disposing a radiating conductor and a ground conductor plate opposite to each other, with an insulating substance interposed therebetween. The shape of the radiating conductor is not particularly limited but, in general, it is rectangular or circular (refer to, for example, Japanese Patent Laid-open No. 2003-304115).
FIG. 10 shows a configuration example of a patch antenna. The patch antenna shown in the figure is composed of a ground conductor plate 1001 and a radiating conductor 1002, and the radiating conductor 1002 is disposed on the upper side of and at a distance from the ground conductor plate 1001. The device dimensions 10a and 10b of the radiating conductor 1002 of the patch antenna are ordinarily not more than one half (½) of the wavelength λ in the frequency band used, whereby a unidirectional radiation pattern can be realized without separately providing a reflector plate.
In the figure, reference numeral 1003 denotes a support for the radiating conductor 1002, which is located at a central portion of the radiating conductor 1002. Reference numeral 1004 denotes a feeder port of the radiating conductor 1002. For excitation, the feeder port 1004 is located with a small offset from the central portion 1003 of the radiating conductor 1002, and matching of the antenna to a desired impedance can be obtained by adjusting the offset length.
In general, the radiating conductor 1002 of the patch antenna is square in shape, the resonance frequency f0 thereof depends on the device dimension 10b of the radiating conductor 1002, and the bandwidth thereof depends on the device dimension 10a. The resonance frequency f0 is not markedly changed even when the device dimension 10a is varied so as to contrive a reduction in the size of the square patch antenna insofar as the variation is within the range for satisfying the bandwidth required of the system.
Since a patch antenna shows a unidirectional directivity generally in the Z-axis direction and a directional gain of a few dBi can be obtained, it is considered that a patch antenna can be favorably applied to the back scatter communication system for carrying out reflected wave transmission, from the viewpoint of obtaining a sufficient signal magnitude. However, in the back scatter communication system, transmission and reception on the reflected wave reader side are conducted in the same frequency band (as above-mentioned), so that there is a need to secure isolation between a transmitting unit and a receiving unit.